Did you tell the agency what you were currently earning? They should be doing the negotiations on your behalf on your salary. You should tell them what you are currently on and what you have been offered. You can always try to bridge the gap eg. if the employer is will to go up by $2K then you will drop by $2K and then it isn't such a big drop. Alternately you could ask for your salary to be reviewed after the probation period so that you can demonstrate that you can do the job then they can give you a pay increase.